1. Field of the Invention
This invention relates to input/output circuits for industrial control systems. In particular, this invention relates to industrial control systems having input/output circuits with programmable input/output characteristics.
2. Description of Related Art
Industrial control systems are widely used for controlling industrial processes. Modern industrial control systems typically comprise a microprocessor-based controller that controls the output status of a plurality of output devices based on the input status of a plurality of input devices. In a widely used arrangement, the microprocessor-based controller is provided in the form of a rack-mounted programmable controller processor module that cooperates with one or more rack-mounted input/output (I/O) modules to communicate with the input and output devices.
With respect to the output devices, typically, the processor module provides the I/O module with a digital representation of the desired status of an output device. For a digital output device (e.g., a solenoid, valve, relay, and so on), the digital representation simply indicates whether the output device should be "on" or "off". For an analog output device (e.g., a heater element in a temperature control system), the digital representation indicates not only whether the output device should be turned on, but also the extent to which the output device should be turned on.
An I/O module comprises an output circuit that connects the I/O module to the output device. In general, the purpose of an output circuit is to provide signal conditioning. The signal conditioning generally involves converting a digital or analog representation of the desired output status of an output device to an output control signal that is capable of driving the output device in the desired manner. For a digital output device, the output circuit converts the low power digital signal indicative of the desired on/off status into a digital output control signal that has the voltage and current characteristics necessary to drive the digital output device.
For an analog output device, the I/O module typically includes a digital-to-analog (D/A) converter that converts the digital representation of the desired output status into an analog representation of the desired output status. The output circuit then converts the analog representation into an analog output control signal having the signal characteristics expected by the analog output device. For example, some output devices are controlled by controlling the current that flows through the output device. Therefore, it is necessary to produce an amount of output current that varies in accordance with the amount of the analog voltage that is produced by the D/A converter. Additionally, it is often necessary to provide level shifting and/or scaling. For example, the D/A converter may be a monopolar device that outputs a voltage which varies between +1 volts and +4 volts, whereas the output device may be a bipolar device that expects an analog output control signal which varies between -10 volts and +10 volts. Therefore, the output circuit provides the necessary level shifting and scaling to convert the 1-4 volt signal to .+-.10 volt signal.
Likewise, for the input devices, input circuits are provided that provide signal conditioning for similar reasons. In particular, an analog input device produces an analog input status signal that indicates the input status of the input device. The analog input status signal usually must be converted to another signal format that can be digitized using an analog-to-digital (A/D) converter, since the A/D converter expects an input voltage signal that varies within a fixed voltage range determined by the A/D converter. Depending on the input device, the analog input status signal may vary between a variety of different possible ranges, and may also be a current signal instead of a voltage signal. Thus, it may be necessary to convert the analog input status signal from a current signal to a voltage signal and/or to perform level shifting and scaling to the analog input status signal so that it may be digitized by the A/D converter.
Typically, an I/O module is provided as a multi-channel device, i.e., the I/O module has multiple output channels and/or multiple input channels. For example, I/O modules are sold that have four output channels and eight input channels. In a multiple channel device, one output circuit is provided for each one of the output devices and one input circuit is provided for each one of the input devices. Thus, in the above example, four output circuits and eight input circuits are provided.
Generally, existing I/O circuits are of limited flexibility. For example, analog I/O modules have a fixed number of input channels and a fixed number of output channels. Therefore, an I/O module with four output channels and eight input channels cannot be reconfigured so as to have six output channels and six input channels. It would be highly advantageous to provide an I/O module that is reconfigurable so that the relative number of output channels and input channels may be varied. It would be even more advantageous to provide an I/O module in the manner just described, and wherein each I/O circuit always uses the same device-side I/O terminal to connect with I/O devices, regardless whether the I/O device is an input device or an output device.
Additionally, existing I/O circuits are limited in their ability to switch between providing a high current/low voltage output signal and low current/high voltage output signal. While a limited amount of flexibility has been achieved to the extent that selectable voltage and current ranges have been achieved, flexibility has still been limited to the extent that an efficient way has not been provided to reconfigure an output circuit between providing a high current/low voltage output signal and a low current/high voltage output signal. The ability to reconfigure an I/O module in this manner would be highly advantageous.